Thrist

Recall that the spelling of English words was fluid up until printed texts
became widespread: Spelling followed pronunciation.
At some point, probably Dr. Samuel Johnson's 1755dictionary, spelling became fixed.

I am informed1 that the above is not correct: Apparently some vowel+r constructs in Old English were pronounced either way, in a phemonenon called "metathesis": We see the final uses of a form that did not survive.

Then gan the Pilgrim thus, I chaunst this day,
This fatall day, that shall I euer rew,
To see two knights in trauell on my way
(A sory sight) arraung'd in battell new,
Both breathing vengeaunce, both of wrathfull hew:
My fearefull flesh did tremble at their strife,
To see their blades so greedily imbrew,
That drunke with bloud, yet thristed after life:
What more? the Redcrosse knight was slaine with Paynim knife.

And then , Book 1, Canto 10, Stanza 38:

The second was as Almner of the place,
His office was, the hungry for to feed,
And thristy giue to drinke, a worke of grace:
He feard not once him selfe to be in need,
Ne car'd to hoord for those, whom he did breede:
The grace of God he layd vp still in store,
Which as a stocke he left vnto his seede;
He had enough, what need him care for more?
And had he lesse, yet some he would giue to the pore.

I found another use of the word "thrist" that isn't certainly a misspelling of "thirst", in the transliteration of a scanned3 Renaissance book printed in the 15th century by the firm of Chepman and Myllar.

This book contains a poem The Testament of Mr Andro Kennedy,
by Scots poet William Dunbar, who lived from c.1460 to c.1520.
Dunbar writes in Scots dialect, and there are a few places where "thrist" appears: